Microscope apparatus and method for phase image acquisition

ABSTRACT

A microscope apparatus includes an electromagnetic wave source configured to generate an illuminating electromagnetic wave, a first beam splitter configured to split the illuminating electromagnetic wave into a first component along a first path and a second component along a second path, a movable reflector module configured to adjust a portion of the second path, and a second beam splitter configured to recombine the first component and the second component. An observing device is configured to receive the recombined first component and second component and the microscope apparatus is configured acquire a phase image from the observing device based on positioning of the movable reflector module and representative of an electric field distribution near an object located along the first path between the first beam splitter and the second beam splitter.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No.13/328,591, filed Dec. 16, 2011, which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to a microscope.

BACKGROUND

A microscope system is used to acquire the intensity images of objects.For integrated circuit fabrication, a photomask image is acquired usinga microscope to predict patterns to be formed in a resist layer on awafer.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an exemplary microscope apparatus forphase image acquisition according to some embodiments;

FIG. 2 is a set of exemplary phase images acquired using the microscopeapparatus in FIG. 1;

FIGS. 3A and 3B are exemplary intensity plots used to extract phaseinformation; and

FIG. 4 is a flowchart of an exemplary method of acquiring phase imagesusing the microscope apparatus in FIG. 1 according to some embodiments.

DETAILED DESCRIPTION

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare illustrative of specific ways to make and use, and do not limit thescope of the disclosure.

In addition, spatially relative terms, for example, “lower,” “upper,”“horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top,”“bottom,” etc. as well as derivatives thereof (e.g., “horizontally,”“downwardly,” “upwardly,” etc.) are used for ease of the presentdisclosure of one features relationship to another feature. Thespatially relative terms are intended to cover different orientations ofthe device including the features.

FIG. 1 is a schematic diagram of an exemplary microscope apparatus forphase image acquisition according to some embodiments. The microscopeapparatus 100 includes an electromagnetic wave source 102 (“source”),e.g., a point source with off-axis illumination (OAI)/free formillumination (FFI), a coherent source, or any other suitable source. Theilluminating electromagnetic wave from the source 102 is not limited to(visible) light, but the terms “light” and “optical” are used asexamples in the following for illustration. Also, the microscopeapparatus 100 acquires phase images of a photomask 120 (“mask”) in oneexample. However, the microscope apparatus 100 can be used to acquirephase images of other objects.

A polarizer (polarizing filter) 104 polarizes the (illuminating) lightfrom the source 102. For example, light passing through the polarizer104 oscillates in only one direction, and is referred to as polarizedlight (and in at least one embodiment, a linear polarized light). Acondenser lens 106 concentrates the light from the source 102 to providea relatively (or generally) homogeneous illumination. Beam splitters 108and 122 are used to split or recombine/rejoin the light.

In some embodiments, the source 102, the polarizer 104, the condenserlens 106, the beam splitters 108 and 122, and the objective lens 124,the NA turn table 126, the Bertrand lens 128, the tube lens 130, and thecamera 132 are aligned along an axis 105 and these elements are held byany suitable housing or clamping structures.

A movable reflector module 110 includes reflectors 112 and 116, and adispersion compensator plate 114. After the illuminating light ispolarized by the polarizer 104, the light is split by the beam splitter108 before the mask 210 and to recombine the split light after the mask210 by the optical elements such as the beam splitter 122 and reflectors112 and 116. The dispersion compensator plate 114 reduces dispersioninduced by optical path difference for different electromagnetic wavefrequencies. Dispersion is caused by different group/phase velocity ofthe light depending on its frequency. A shutter 118 opens or closes thelight path through the movable reflector module 110. For example, theshutter 118 can be opened to allow the light to travel on the light path136 from the beam splitter 108 to reflectors 112 and 116, and to thebeam splitter 122. A focusing lens 119 transfers a parallel light into abeam which contains all oblique angles of light by focusing.

An objective lens 124 magnifies and projects the (illuminating) lightafter the mask 120 that has the mask pattern towards an observing devicesuch as a camera 132, e.g., a charge-coupled device (CCD) camera. Thefocusing lens 119 can be similar to the objective lens 124 in a reversedirection in some embodiments. A numerical aperture (NA) turn table 126adjusts the NA of the objective lens 124. NA is the range of angles overwhich the system can accept or emit light such that the NA of a beam isconstant as the beam goes from one material to another provided there isno optical power at the interface. NA is a measure of the ability togather light and resolve fine specimen detail at a fixed objectdistance. An exemplary NA range is 0.9-1.4.

A Bertrand lens (phase telescope) 128 is moveable to get images of theNA plane (pupil plane, Fourier plane). The Bertrand lens 128 is anoptical device used in aligning the various optical components of amicroscope. In particular it allows observation of the back focal planeof the objective lens 124 and its conjugate focal planes. The Bertrandlens 128 moves the intermediate image plane to a point where it can beobserved. A tube lens 130 is placed before the camera 132 in themicroscope apparatus 100, for the purpose of providing a paralleloptical path. Images will be projected on the camera 132 after the tubelens 132 to obtain the phase images.

The electric field distribution E_(o) on the pupil plane at theobjective lens 124 is referred to as a near field since it is close tothe mask 120. In some examples, the distance between the near fieldimage region to the mask 120 is less than a few wavelengths of theilluminating light. A latent image is an image from the electric fielddistribution E_(l) at the camera 132. The phase image acquisition fromthe mask 120 using the microscope apparatus 100 in FIG. 1, allowsreconstructing the near-field image, which helps to precisely predictthe latent image on a resist layer of a wafer for real integratedcircuit fabrication.

A moving mechanism 134 (e.g., a step motor) moves the movable reflectormodule 110 by a controlled distance (Δz) to change the optical pathlength 136 between the beam splitters 108 and 122 via the reflectors 112and 116. When the position (z) of the movable reflector module 110 ismoved by Δz, the optical path length 136 is changed by 2Δz. At least twoimages obtained with the movable reflector module 110 at two differentrespective z-positions are used to obtain phase information. In at leastone embodiment, more detailed phase information is obtained withmultiple images from multiple positions of the movable reflector module110.

For example, a phase image can be acquired using the camera 132 at aninitial position (Δz=0), then at the second position (Δz=λ/8), and atthe third position (Δz=λ/4), where λ is the wavelength of theilluminating light. Exemplary phase images in FIG. 2 are acquired usingthe microscope apparatus 100 in FIG. 1 with intensity variation of eachpixel at the (CCD) camera 132. In FIG. 2, a phase image 202 is withΔz=0, a phase image 204 is with Δz=λ/8, and a phase image 206 is withΔz=λ/4.

FIGS. 3A and 3B are exemplary intensity plots used to extract phaseinformation. FIG. 3A is an intensity plot with Δz=0. The intensity plotsare obtained from phase images such as shown in FIG. 2 along a z-axisthrough the center. The intensity can be described by the followingequation:Intensity=|E ₀Exp(if)+E _(r)Exp(−i4πΔz/λ)|²  (Equation 1),where E₀ is a real number light field, Er is a complex number lightfield, f is phase, z is the position of each sampling point, and λ isthe wavelength of the light. FIG. 3B is an intensity plot with differentz.

By measuring the relative intensity of three sampling points 302 in FIG.3A and three sampling points 304 in FIG. 3B, the phase f could bedetermined from Equation (1). More phase images (e.g., three or more)with different z-positions make phases determination more precise andthe trends of intensity variations can be more accurately described.

The microscope apparatus 100 is an imaging system to collect the lightfield from an object (such as a mask 120) to an image plane of thecamera 132. The light fields from different light paths interfere witheach other at the pupil plane of the NA turn table 126 (from the twolight paths split at the beam splitter 108 and recombined/rejoined atthe beam splitter 122). As described above, the phase information of thelight field can be obtained from the phase images such as FIG. 2.

The phase information can be used to reconstruct the image at aphotoresist for a real integrated circuit fabrication when the mask 120is used. Such image reconstruction is used for advanced integratedcircuit technology nodes as the integrated circuit scale shrinks, suchas 20 nm technology and beyond, because the image variation through thedirection of imaging depth is much larger than previous technologynodes. With an image reconstruction method including the phaseinformation, the photoresist image can be more accurately obtained.

The electric field distribution E_(o) on the pupil plane at theobjective lens 124 is referred to as a near field since it is close tothe mask 120. In some examples, the distance between the near fieldimage region to the mask 120 is less than a few wavelengths of theilluminating light. A latent image is an image from the electric fielddistribution E_(l) at the camera 132. The phase image acquisition fromthe mask 120 using the microscope apparatus 100 in FIG. 1, allowsreconstructing the near-field image, which helps to precisely predictthe latent image on a resist layer of a wafer for real integratedcircuit fabrication.

FIG. 4 is a flowchart of an exemplary method of acquiring phase imagesusing the microscope in FIG. 1 according to some embodiments. At step402, illuminating electromagnetic wave is split into a first path and asecond path at a first location. At step 404, the illuminatingelectromagnetic wave is recombined at a second location where the firstand second paths rejoin. At step 406, an object to be observed, such asa photomask for integrated circuit fabrication, is loaded in the firstpath between the first and the second locations. At step 408, the firstphase image is acquired. At step 410, the length of the second path ischanged. At step 412, the second phase image is acquired after changingthe length of the second path.

Before or during acquiring phase images following the above steps, othersteps and adjustments may be performed. For example, the object such asthe photomask is loaded in the microscope. The object is aligned for themicroscope. In some applications, a C/R (clear reference) position isdefined on the object (mask) for acquiring a clear reference imagewithout features. The microscope may automatically adjust the for C/Rposition during an automated process, then acquire a clear referenceimage. A position for an objective lens is adjusted for focus. Theposition of the condenser lens is adjusted to have even illumination onthe object. For example, a condenser position is achieved by focusingthe image on an observing device such as a CCD camera. The NA is alignedto be centered in the beam path on the pupil plane. The right exposuretime is determined for each type of illumination settings to achievegood intensity level on the acquired images from the camera for thedesired settings.

The above method embodiment shows exemplary steps, but they are notnecessarily required to be performed in the order shown. Steps may beadded, replaced, changed order, and/or eliminated as appropriate, inaccordance with the spirit and scope of embodiment of the disclosure.

In some embodiments, a microscope apparatus includes an electromagneticwave source configured to generate an illuminating electromagnetic wave,a first beam splitter configured to split the illuminatingelectromagnetic wave into a first component along a first path and asecond component along a second path, a movable reflector moduleconfigured to adjust a portion of the second path, and a second beamsplitter configured to recombine the first component and the secondcomponent. The microscope apparatus also includes an observing deviceconfigured to receive the recombined first component and secondcomponent and the microscope apparatus is configured acquire a phaseimage from the observing device based on positioning of the movablereflector module and representative of an electric field distributionnear an object located along the first path between the first beamsplitter and the second beam splitter.

In some embodiments, a method of acquiring a phase image includessplitting an illuminating electromagnetic wave into a first componentalong a first path and a second component along a second path at a firstlocation, the second path being set based on a movable reflector module.The method further includes recombining the first component and thesecond component at a second location where the first path rejoins thesecond path, loading an object to be observed in the first path betweenthe first location and the second location, and acquiring, from anobserving device, a phase image, wherein the phase image is based onpositioning of the movable reflector module and representative of anelectric field distribution near the object.

In some embodiments, a method of acquiring phase information includessplitting an illuminating electromagnetic wave into a first componentalong a first path and a second component along a second path at a firstlocation, the second path being set based on a movable reflector module.The method further includes recombining the first component and thesecond component at a second location where the first path rejoins thesecond path, loading an object to be observed in the first path betweenthe first location and the second location, acquiring, from an observingdevice, three intensity measurements at each of two positions of themovable reflector module, each measurement representative of an electricfield distribution near the object, and extracting the phase informationfrom the intensity measurements.

A skilled person in the art will appreciate that there can be manyembodiment variations of this disclosure. Although the embodiments andtheir features have been described in detail, it should be understoodthat various changes, substitutions and alterations can be made hereinwithout departing from the spirit and scope of the embodiments.Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, and composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the disclosed embodiments, processes, machines,manufacture, compositions of matter, means, methods, or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein may be utilized according to the presentdisclosure.

What is claimed is:
 1. A method of acquiring a phase image, the methodcomprising: splitting an illuminating electromagnetic wave into a firstcomponent along a first path and a second component along a second pathat a first location, wherein the first path has a fixed length, and thesecond path has an adjustable length being set using a movable reflectormodule; recombining the first component and the second component at asecond location where the first path rejoins the second path; loading anobject in the first path between the first location and the secondlocation; and acquiring, from an observing device, a phase image of theobject, wherein the phase image is based on positioning of the movablereflector module and representative of an electric field distributionnear the object.
 2. The method of claim 1, wherein loading the object inthe first path comprises loading a photomask for integrated circuitfabrication.
 3. The method of claim 1, wherein the phase image isrepresentative of the electric field distribution at a distance from theobject that is less than three wavelengths of the illuminatingelectromagnetic wave.
 4. The method of claim 1, wherein the positioningof the movable reflector module is based on a wavelength of theilluminating electromagnetic wave.
 5. The method of claim 1, furthercomprising obtaining phase information based on three sampling pointsfrom an intensity plot obtained from the phase image.
 6. The method ofclaim 1, wherein acquiring the phase image is based on intensityvariations at an image plane of the observing device.
 7. The method ofclaim 1, further comprising defining a location on the object foracquiring a clear reference image.
 8. A method of acquiring phaseinformation, the method comprising: splitting an illuminatingelectromagnetic wave into a first component along a first path and asecond component along a second path at a first location, wherein thefirst path has a fixed length, and the second path has an adjustablelength being set using a movable reflector module; recombining the firstcomponent and the second component at a second location where the firstpath rejoins the second path; loading an object in the first pathbetween the first location and the second location; acquiring, from anobserving device, three intensity measurements of an intensity plot ofthe object at each of two positions of the movable reflector module,each measurement representative of an electric field distribution nearthe object; and extracting the phase information of the object from theintensity measurements.
 9. The method of claim 8, wherein loading theobject in the first path comprises loading a photomask for integratedcircuit fabrication.
 10. The method of claim 9, further comprisingreconstructing an image at the photomask based on the phase information.11. The method of claim 8, wherein each measurement is representative ofthe electric field distribution at a distance from the object that isless than three wavelengths of the illuminating electromagnetic wave.12. The method of claim 8, further comprising determining the twopositions of the movable reflector module based on a wavelength of theilluminating electromagnetic wave.
 13. A method of reconstructing anear-field image of an object, the method comprising: positioning theobject between a first location and a second location; illuminating theobject with a first portion of an electromagnetic wave along a firstpath; redirecting, at the first location, a second portion of theelectromagnetic wave along a second path, using a movable reflectormodule; rejoining, at the second location, the first portion of theelectromagnetic wave and the second portion of the electromagnetic wave;directing the rejoined electromagnetic wave to an objective lensconfigured to magnify the rejoined electromagnetic wave; and obtaining aphase image of the object using an observing device, wherein the phaseimage is based on positioning of the movable reflector module andrepresentative of an electric field distribution near the object. 14.The method of claim 13, wherein positioning the object between the firstlocation and the second location comprises positioning a photomask forintegrated circuit fabrication between the first location and the secondlocation.
 15. The method of claim 13, further comprising reconstructingthe near-field image of the object using the phase image of the object.16. The method of claim 13, further comprising defining a location onthe object for acquiring a clear reference image.
 17. The method ofclaim 13, further comprising projecting a latent image on the observingdevice, the latent image representing the electric field distributionnear the object.
 18. The method of claim 13, wherein the redirecting ofthe second portion of the electromagnetic wave comprises splitting theelectromagnetic wave into the first portion of the electromagnetic waveand the second portion of the electromagnetic wave using a beamsplitter.
 19. The method of claim 13, further comprising using a movingmechanism to move the movable reflector module by a controlled distance.20. The method of claim 13, further comprising adjusting a numericalaperture (NA) of the objective lens using a NA turn table.